Plant salt tolerance and drought resistance are key factors in agriculture and environmental conservation. About one-third of the Earth is presently regarded as dry land. Desertification of farmland and green space is progressing, and the percentage of dry land is expected to increase in the future. Considering that the world's population in 2050 is estimated to be over 150% of the present population, and the increasingly serious food problem, the development of cultivation techniques and crop varieties that grow on unfavorable land, especially on dry land, are matters of urgency. Salinization is a serious problem in farming dry land. Since evapotranspiration is greater than rain fall in dry climates, continuing irrigation in poor drainage conditions promotes the rising of salt-containing groundwater levels and the deposition of salts at the soil surface. As a result, excessive amounts of salt accumulate in the soil. Cases where agriculture was abandoned due to salt accumulation, such as the decline of the Tigris-Euphrates Civilization, have been known since ancient times, and even today, salt accumulation is often problematic. Thus, enhancing salt and drought tolerance in crops is an important challenge in the advancement of agriculture in dry and salinity-affected land (Tadano, T. (1983) Salt tolerance in crops and its mechanisms. Kagaku to Seibutsu 21, 439-445; Uchiyama, Y. (1988) Agricultural utilization of high-salt environments. Kagaku to Seibutsu 26, 650-659.).
If a gene responsive to a stress such as salt or drought can be isolated, the introduction of this gene to a given crop cultivar, by transformation methods or such, may enhance the cultivar's tolerance to salt and drought stresses in such systems.
The plant parts that are above ground, such as leaves and shoot apices, clearly play important roles in the environmental responses of plants. Roots too play a crucial part in stress tolerance, since, in conjunction with water and nutrient absorption, roots have regulatory mechanisms that respond to stresses such as drought, salt and low temperature.
Therefore, the isolation of genes in plant roots that respond to salt or drought stress was desired.